The present invention relates to digital key telephone desksets, and more particularly, to a method and circuit for canceling a tone signal noise generated in using a speakerphone deskset.
Typically, in digital key telephone deskset, a dual tone multifrequency signal (hereinafter referred to as a "DTMF" signal) is generated in correspondence with the different ones of the number keys of a keypad pressed by a user when using a speakerphone deskset, and noise that is input via the microphone of the speakerphone are concurrently transmitted to the key telephone main equipment. If the user presses number keys using the keypad of the deskset, the microprocessor controls a tone generator included in telephone communication unit to generate the DTMF signal corresponding to the pressed number keys. The DTMF signal is transmitted to a data transmitter via network controller and coder. The data transmitter transmits the DTMF signal to the key telephone main equipment together with a channel D among 2B+D channels. Since the user uses a speakerphone, a communication path including the microphone of the deskset is formed. Thus, in using the speakerphone, the noise input through the microphone is transmitted to the data transmitter via switch, an analog-to-digital converter, a transmitting bandpass filter, network controller and coder. The data transmitter transmits the noise to key telephone main equipment together with a channel B among 2+D channels.
Therefore, the DTMF signal and the noise are mixed in the network formed when the key telephone main equipment transmits the DTMF signal to another exchange system via a central office line. As the result, a DTMF receiver of another exchange system for receiving the DTMF signal from the central office line can not precisely analyze the DTMF signal due to the noise. The noise input to the microphone of the digital key telephone apparatus is the sounds, i.e., the noise, generated when the number keys are pressed. The tone signal noise generated when the user presses the number keys. The DTMF signal is not intense so as to be negligible while keys are being pressed. The DTMF signal is however, severely affected by the noise when the keys are pressed into an electrically conducting, or "on", state or released to an electrically non-conducting, or "off", state. Since the noise is maintained for a predetermined time period ta when a key is released (no longer pressed), the quality of the DTMF signal is considerably deleteriously affected.